Photochromic molecules are useful for a variety of research and commercial applications in fields ranging from sunglasses to memory storage devices. A myriad of configurations have been developed, seeking to obtain improvements in stability, control in switching, fatigue resistance, sensitivity and the like. Diarylethenes have found favour for several of these traits, and are the subject of continued investigation. A review by Irie (Proc. Jpn. Acad. Ser B 86:472-483, 2010) illustrates a range in stability, colour and the like of selected diarylethenes.
PCT Publication WO2004/015024 describes compounds that are both photochromic and electrochromic, and methods of making such compounds, and describes a mechanism of catalytic electrochromism. Briefly, a ring-closed form (isomer B) of a compound loses an electron under electrochemical conditions, forming a radical cation. A rapid ring-opening reaction occurs, providing the radical cation of isomer A, which oxidizes a neighbouring compound of isomer B, neutralizing the radical cation. This ring opening reaction may be initiated with a small charge, and perpetuates throughout the material, resulting in conversion of the ring-closed isomers to the ring-open isomers. PCT Publication WO2010/142019 describes variable transmittance optical filters comprising a material capable of transitioning between light and dark states in response to ultraviolet light and electric voltage, the material comprising a chromophore that has both electrochromic and photochromic properties.
Light transmission properties of such optical filters may be varied by selection of a photochromic-electrochromic diarylethene with greater or lesser light absorbance in the ring-open or ring-closed form. To provide for such variation, there is a need for molecules with improved photochromic, electrochromic or photochromic and electrochromic properties.